Modelling the impact of different irrigation regimes and mulching on strawberry crop growth and water use in the arsenic-contaminated Bengal basin

Replacement of water-intensive winter rice with strawberry (Fragaria × ananassa Duch.) may restrict groundwater extraction and improve water productivity and sustainability of agricultural production in the arsenic-contaminated Bengal basin. The potential of strawberry cultivation in terms of yield obtained and water use efficiency need to be evaluated under predominant soil types with mulch applications. Water-driven model AquaCrop was used to predict the canopy cover, soil water storage and above-ground biomass of strawberry in an arsenic-contaminated area in the Bengal basin. After successful calibration and validation over three seasons, AquaCrop was used over a range of management scenarios (nine drip-irrigation × three soil types × four mulch materials) to identify the best irrigation options for a drip-irrigated strawberry crop. The most appropriate irrigation of 176 mm for clay loam soil in lowland and 189 mm for sandy clay loam in medium land rice areas and the use of organic mulch from locally available jute agrotextile improved 1.4 times higher yield and 1.7 times higher water productivity than that of without mulch. Strawberry can be introduced as an alternative crop replacing rice in non-traditional upland and medium land areas of the arsenic-contaminated Bengal basin with 88% lower groundwater extraction load and better economic return to farmers.


Model calibration
The initial (CC o ) and maximum (CC m ) canopy cover by strawberry with a population of 5.3 m −2 were 0.80 (80%) and 95% respectively at 1.0ETc under the straw mulching.Calibrated coefficients of canopy growth (CGC) and canopy decline (CDC) were measured as 10.4% d −1 and 8.0% d −1 , respectively.Average soil moisture at the active root zone (0.40 m) was around two times higher at drip-irrigated mulched plots over unmulched surface irrigated one (Supplementary Fig. S3).The calibrated aboveground biomass WP value was adjusted to 17 g m −2 , which was within the model-recommended range of 15-20 g m −2 for C3 plants.The value of the crop sink-strength coefficient (f sink ) was set at 40% 37,38 , while the value of the reduction coefficient for the products synthesized (f yield ) was taken as 50%.The reference harvest index (HI o ) for the Sweet Charlie variety in the study was estimated at 35% 39 .
The threshold values of soil water depletion were adjusted at 0.20 to 0.55 for canopy expansion, 0.50 for stomatal conductance, and 0.65 for canopy senescence as per model guidelines 35,40 .These adjustments were made keeping in mind the shallow root distribution of strawberry under mulch and drip irrigation and making the growth sensitive to the soil water (Table 1).

Canopy cover
The model satisfactorily simulated the seasonal trend in canopy cover in all treatments for two consecutive seasons with r 2 of 0.94, RMSE of 4.8%, and d of 0.94 (Supplementary Fig. S5 and Table 3).The model captured the variability of canopy cover in all the irrigation-mulch combinations except during peak vegetative stage to maturity (45-140DAP) in 0.8ETcNM, 0, 8ETcBM and 1.0ETcJM in 2016-17 (Supplementary Fig. S4a) and in 0.8ETcNM, 1.0ETcSM and 1.0ETcBM during early-to-peak flowering stage (45-75DAP) in 2017-18 (Supplementary Fig. S4b).In all these cases, model values were 6.5% lower than the field-measured values but the canopy senescence was appropriately estimated in all the treatments during both seasons.

Irrigation and mulching scenarios analysis
Optimizing the irrigation amount for strawberry requires an understanding of the response of berry productivity (ET, yield, and WP) to irrigation amount.On the other hand, the response also depends on the variability in mulch type and soil texture.Here 108 numerical simulations (9 irrigation amounts × 3 initial soil types × 4 mulch materials) were conducted to select the appropriate irrigation amounts under various mulching and soil types.

Response of ET to irrigation amount
The ET was directly proportional to irrigation to a maximum level (the maximum ET, ET m ) for all soil types irrespective of the mulches used (Supplementary Fig. S7).Further increment in irrigation water has not been effective for strawberry, indicating a threshold value for maximum economic return.

Response of strawberry yield to irrigation amount
The responses of strawberry yield to irrigation amount under various soil and mulch types are reported in Supplementary Fig. S9.The relationship was parabolic with r 2 values of 0.73, 0.76 and 0.85 for clay, clay loam and sandy clay loam soils, respectively.The yield of berries increased to a maximum at irrigation amount of 165 mm for clay soil, 178 for clay loam soil and 190 mm for sandy clay loam soil.In furtherance of irrigation water application, the fruit yield started declining.The highest yield was simulated in the sandy clay loam with 189 mm of drip irrigation under jute agrotextile mulch (Supplementary Fig. S8).
Strawberry yield differed among soil types (p < 0.001) or the use of mulches (p < 0.05) (Table 5).In general, the yield was highest in sandy clay loam and the lowest in clay soil.The effect of mulches in terms of yield followed the order of JM > BM > SM > NM.The highest yield was predicted with JM in sandy clay loam soil (1.82 t ha −1 ) followed by BM in sandy clay loam soil (Supplementary Fig. S8), while, the lowest yield was predicted in unmulched clay soil.

Response of WP to irrigation amount
The WP and irrigation amount had typical parabolic relationships (Supplementary Fig. S9).The WP increased sharply with the increase in yield in response to higher irrigation water use.The yield reached a maximum at the average 200 mm irrigation over soil and mulch types, after which it decreased as more water was applied which lowered the WP.The WP was the highest in sandy clay loam.BM-use checked soil evaporation most effectively in comparison to other mulches and resulted in the highest WP.The interactive effect of soil and mulch type predicted the highest WP in sandy clay loam-JM followed by in sandy clay loam-BM followed by clay loam-JM.

Appropriate irrigation amount
Optimum irrigation amounts for strawberry ranged between 160 and 200 mm under the test soils and mulch types to minimize loss of water and ensure higher yield and WP.Simulated evapotranspiration (ET m ) and water Table 1.Date of planting and irrigation scheduling of strawberry in different years. 1 Drip irrigation to meet 100% (1.0ETc), 80% (0.8ETc) and 60% (0.6ETc) crop evapotranspiration (ETc), respectively, under the standard condition 67 [ETc = E-pan × Pan coefficient (Kp, 0.85) × crop coefficient (Kc)]; *Kc values were taken from FAO manual No. 56 67 . 2 Includes 25 mm water for field preparation and establishment of seedlings.6).The best yield and WP were achievable with ~ 189 mm irrigation and the use of nonwoven jute agrotextile mulch in sandy clay loam soil (Fig. 2).The differences were most likely caused by different soil textures, and mulch types, that determined the growth and development behaviour of below and above-ground plant parts.The performance of JM was also better compared to other mulches in terms of maximum and appropriate yield, WP and ET.The most optimal irrigation quantities were also found to be greater for sandy clay loam soil than clay loam or clay soil, indicating that strawberry was water sensitive and needed a suitable air-water ratio to ensure both aeration and soil water supply.The ET corresponding to the most appropriate irrigation amount ranged as 126-159, 144-180 and 163-205 mm for clay, clay loam and sandy clay www.nature.com/scientificreports/loam soil.Among all soil-mulch combinations, the highest yield was obtained with the jute agrotextile mulch in sandy clay loam (2.78 t ha −1 ) followed by in clay loam soil (2.11 t ha −1 ) with applications of 189 and 176 mm of drip irrigation, respectively.

Discussion
Calibrated parameters indicated good agreements between observed and model-simulated data as reflected from the r 2 values for canopy cover, soil-water-storage in the root zone, aboveground biomass, and yield.However, lesser average root depth than that recommended by the model for strawberry could be attributed to the application of mulch and drip irrigation, which kept the upper soil layers moist.Soil water content and root length density were observed to be higher in the topsoil layers and the relatively lower in the deeper layer in other studies also 41,42 .The accuracy of the model is also reported in different crops (in soybean 43,44 , potato 45 , tomato 46 , sugar beet 47 , amaranthus 48 and cabbage 49 ) under different hydro-meteorological conditions globally.Model values for canopy cover were 6.5% lower than the field-measured values, indicating a need for further understanding the weather-plant interactions in the model for simulating root growth, moisture utilization and aboveground canopy expressions.However, the canopy senescence was appropriately estimated.However, the model overestimated soil water storage marginally in jute agrotextile mulch.There could be higher soil water uptake by the crop with enhanced root activity and this perforated textile mulch does not cover the soil surface entirely like plastic mulches, and therefore the chance of surface water escaping to the atmosphere.The extent of moisture loss through soil evaporation and crop transpiration depends on the type of mulches 50,51 .Plastic mulch may reduce evaporation loss by almost sealing the soil-atmosphere interface.The aerobic root zone under jute mulch can enhance root growth 5,52,53 facilitating higher root water uptake, and conserving the residual soil moisture content.Partitioning of evapotranspiration into evaporation and transpiration may be fine-tuned in AquaCrop based on the FAO56 approach 54 .The study demonstrated good agreement with the simulation of the aboveground biomass and is in consistent with previous studies in potato 55 , cabbage 49 , and soybean 44 .However, lower simulated aboveground biomass in this study at maturity under 0.6-0.8ETcirrigation regime irrespective of mulches may Hence the differences could be due to either a lower simulated WP or transpiration underestimating the root water uptake.The model could not pick up the effect of terminal heat stress on the aboveground biomass.This is the limitation of the model for tropical moist sub-humid climate and further work on this aspect is required.
The ET was the highest in sandy clay loam soil and the lowest in clay soil.The influence of soil texture on ET was also explained earlier 56 .The ET was the lowest in BM and the highest in SM in sandy clay loam and clay loam soil.The difference in evaporation and water-retention capacity was observed under different soil types and the use of different mulches 57 .
The relationship between strawberry yield and irrigation amount under various soil and mulch types was parabolic as observed earlier 58,59 .In general, the yield was highest in sandy clay loam and the lowest in clay soil.www.nature.com/scientificreports/This may be due to differences in water retention 60 , nutrient availability 61 and root growth 62 that influenced above-ground biomass and yield in strawberry in favour of sandy clay loam texture.The impact of mulches on yield exhibited the sequence: JM > BM > SM > NM.The most substantial yield was anticipated using JM in sandy clay loam soil, succeeded by BM in the same soil type.Conversely, the minimal yield was projected for unmulched clay soil.Strawberry is a shallow-rooted crop that requires frequent but minimal amounts of water to produce quality berries.Higher water retention in clay soil can lead to inadequate aeration in the root zone and nutrient leaching, reducing crop performance 63 .Straw mulching moderate soil hydrothermal regime favourably increases water productivity, and keeps berries healthy by reducing contact with the soil to avoid fruit-rot; black polyethene mulch facilitates root growth, and nutrient uptake, provide soil aggregate stability although adversely affecting soil ecophysiology 5 .The use of plastic mulch in the Bengal basin may further warm the soil at the root zone resulting in root and leaf senescence and thereby may reduce crop yield and water use 64 .Organic mulch like JM improved the soil microclimate in favour of microbial growth, rooting behaviour and availability of nutrients 5 .Our study confirms that cultivation of drip-irrigated strawberry is favourable both in sandy clay loam and clay loam soils with the use of organic jute agrotextile mulch for higher yield at appropriate irrigation levels.The combined influence of soil and mulch type indicated the peak water productivity (WP) in sandy clay loam -JM, succeeded by sandy clay loam -BM, and subsequently by clay loam -JM.The appropriate irrigation amounts worked out under different soil type and mulch type.Y rel and WP rel responded to irrigation amount similarly to yield and WP, and their interactions could likewise be explained by a quadratic function of irrigation amount 65 .The disparities were likely a result of varying soil textures and mulch varieties, influencing the growth and behaviour of both above-ground and below-ground plant components.Table 6.Simulated evapotranspiration (ET m ) and water productivity (WP) corresponding to the maximum fruit yield (Ym) in strawberry, and the same under the best irrigation practice. 1NM, SM, BM and JM are no-mulch, straw mulch, biodegradable plastic and jute agrotextile mulch, respectively.www.nature.com/scientificreports/Additionally, JM outperformed other mulch types in achieving superior outcomes contributing maximum yield, higher water productivity, and controlling evapotranspiration.Furthermore, the ideal irrigation volumes were observed to be higher for sandy clay loam soil in comparison to clay loam or clay soil.This suggests the water sensitivity of strawberries, emphasizing the need for an appropriate air-water balance to ensure effective aeration and adequate soil moisture availability.

Conclusions
AquaCrop was calibrated using field experimental data (2015-16) and validated with two seasons of data of 2016-17 and 2017-18 to predict canopy cover, soil water storage and above-ground biomass of drip irrigated strawberries in arsenic-affected Bengal basin.Crop and management parameters and their coefficients were adjusted using field data under different irrigation regimes and mulch use.The calibrated model can reliably simulate CC, soil water storage and aboveground biomass in subsequent two seasons in strawberry.Following validation, AquaCrop simulations were run under several treatment conditions (9 irrigation amounts × 3 initial soil types × 4 mulch materials) for drip-irrigated strawberry.The results of these simulations revealed that soil type and mulch had separate and combined effects on ET, yield, and WP.The results on yield, ET and WP revealed that sandy clay loam and clay loam soils are most suitable for strawberry cultivation for upland and medium land, respectively.Simulated irrigation amounts of 176 mm for clay loam and 189 mm for sandy clay loam are the most appropriate.Organic mulch from locally available jute agrotextile may improve 1.4 times higher yield and 1.7 times higher water productivity than that of without mulch in strawberry.The findings of present paper may help all the stakeholders including policymakers to opt for strawberry as alternative more remunerative crop with less water requirement.It can replace groundwater exhaustive winter rice in non-traditional up and medium land of arsenic-affected Bengal basin.

Experimental site
The experiment was conducted in sandy loam soil under a tropical moist sub-humid climate at the Central Research Farm, Bidhan Chandra Krishi Viswavidyalaya, Gayeshpur, India (23°5.3′N, 83°5.3′E; 9.75 m above mean sea level).The experimental site is a part of the larger Bengal basin where arsenic contamination in soils and groundwater is predominant.The annual rainfall is 1600 mm of which 85% is received between the 3rd week of June to the end of September.January is the coldest (15.5 to 21.3℃) (Supplementary Table S2), while May is the hottest month (27.6-31.7 °C).Mean relative humidity remains high (82-95%) from June to October and reduces to 70% in January.Wind speed varies from 0.6 to 6.8 km d −1 .Monthly weather parameters of the growing seasons are given in Supplementary Table S2.
The date of planting and irrigation scheduling under each irrigation regime and irrigation-mulch combination treatment are presented in Supplementary Fig. S1 and Table 1.Cultivation input details are presented in Supplementary Table S3.JM and BM were placed in each plot with 80 mm diameter slits at 350 mm × 300 mm spacing.Micro-propagated plantlets of strawberry were planted on November 4, November 2, and November 1 in 2015, 2016, and 2017, respectively, on raised beds measuring 1100 mm at the base, 700 mm at the top, and 300 mm in height, with 400 mm spacing between two successive beds.The impermeable film was embedded in the soil at a depth of 600 mm between each plot to prevent lateral seepage.www.nature.com/scientificreports/

Meteorological data
Daily meteorological data for rainfall, sunshine hour, maximum and minimum air temperatures, relative humidity, and wind speed were collected from the automatic weather station located approximately 40 m distant from the experimental field.The pan evaporation (Epan, mm) was recorded daily using a standard USWB Class A evaporimeter.The crop reference evapotranspiration (ET0, mm) was computed by multiplying Epan with the pan coefficient (Kp) value.The Kp was taken as 0.85 since the experimental site has > 75% relative humidity, > 2 m s −1 wind speed and 1000 m windward side distance from the pan placed in a short green cropped area 67 .

Measurement of soil-water content
Soil water content (SWC,% v/v) was monitored daily using a PR2/6 profile probe (Delta-T Devices Ltd., Cambridge, UK) from 0-100, 100-200, and 200-300 mm layers before and after each irrigation.Tensiometers were put at 250 and 350 mm soil depths in each plot to estimate drainage from the active root zone (> 90% of the strawberry roots are located within the top 300 mm layer 68 ).

Plant growth parameters
The days after planting (DAP) of first runner development, flowering, fruiting and maturity were recorded.Three plants were chosen at random from each plot to measure the maximum length and width of green leaves every 10-15 days during the growing season.The individual plants were cut above the ground at 30 days intervals, and the area of the leaves was determined by using an AM 300 leaf area meter (ADC Bio Scientific Ltd., UK) and the leaf area index (LAI) was computed accordingly.The canopy cover (CC) was estimated by using the following equation 69 : Three plants were oven-dried at 70 °C to constant weights 70 to determine the aboveground biomass.Strawberry fruits were harvested at maturity.There were 13 pickings in the first year and 11 harvests in the second and in the third year at 2 to 7 days intervals.Ten innermost plants from each plot were tagged for recording yield (g plant −1 ) at maturity.

Model calibration and validation
The AquaCrop model (version 6.1) was selected in this study.Standard values for a few universal parameters in strawberry were considered 71 .Others parameters were required to be adjusted to the local conditions.
Crop parameters for plant development and berry production, as well as soil texture, and hydrological properties were used for model calibration as per the requirement of the model 71 .The slope of aboveground biomass versus normalized transpiration was used to calculate normalized biomass water productivity (i.e., (T r /ET o ), T r represents transpiration).The model was calibrated with measured data for all treatments in the 2015-16 season.The mulch coverage percentage was set at 90% (BM and JM), 60% (SM) and 0% (NM).The phenology of the crop was taken as the average of the three growing seasons: 60DAP for flowering, 70DAP for maximum root depth, 79DAP for maximum canopy, 120DAP for senescence, and 140DAP for maturity.The duration of the flowering and yield formation was kept as 60 and 70 days, respectively.Measured SWC at sowing was taken as the initial input to the model.The calibrations were run in day mode, beginning by fitting the parameters from 1.0ETcSM plots (drip irrigation at 100% evaporative demand and use of straw mulch).The crop parameters at 1.0ETcSM treatment were repeatedly altered until the simulated and observed results (SWS, CC, above-ground biomass, and yield) matched acceptably well for other treatments in 2015-16.The main crop parameters in AquaCrop for drip irrigated and mulched strawberry are presented in Supplementary Table S4.
After the calibration, the model was validated using the measured data sets from the 2016-17, and 2017-18 seasons.The model's outputs were compared to the observed data using the coefficient of determination (r 2 ), root mean square error (RMSE), and the index of agreement (d), which were calculated as: where Oi and Si are the observed and simulated values, respectively; O and S are the average observed and simulated values, respectively; and n is the number of observations.The r 2 represents the proportion of the variance in measured data explained by the model.The average magnitude of the discrepancy between simulations and observations is measured by the RMSE, where a maximum 15% error is acceptable for agronomic studies 72,73 .
(1) www.nature.com/scientificreports/ The index of agreement (d) is a measure of the over-and under-estimations of the model, which can't be judged by r 2 74 .It has been reported that the results from the simulation may be acceptable at r 2 > 0.5 or d > 0.65 75 .

Simulation scenario
After the validation, the model was used to simulate ETc, yield, and WP of strawberry under various irrigation amounts, and to identify the best irrigation and mulch treatment in clay soil (1.2-4.4% sand, 30.5-32.7% silt and 65.1-66.1% clay) of lowland (0-30 m amsl; traditional winter rice area), clay loam (11.0-12.8%sand,35.3-43.3%silt and 43.9-53.7%clay) of medium land (31-60 m amsl; non-traditional upland arsenic contaminated winter rice area) and sandy clay loam (42.4-42.7%sand, 20.7-26.8%silt and 30.5-36.9% clay) of upland (61-100 m amsl; mon-traditional medium land arsenic contaminated area areas of Bengal basin 2 .Unbridled groundwater extraction is prevalent in these two non-traditional rice areas 1,5,76 .The simulations were run using a groundwater depth of 2.0 m.The climate in the year 2000-01 was selected as the representative of the long-term average based on the average Mahalanobis distance obtained from variance-covariance matrices (Supplementary Fig. S2).This data was used to simulate the scenarios in AquaCrop.
A total of 14 irrigation events with the same irrigation frequency and irrigation amount after adjusting the effective rainfall and ETc were set for the simulation of each scenario.The first and the last irrigation was given on 17 and 136DAP, respectively, along with an additional 25 mm water for land preparation and seedling establishment up to 15DAP.A total of 108 numerical simulations (9 irrigation amounts × 3 initial soil types × 4 mulch materials) were carried out.
The analysis of variance was used to investigate the differences in simulated ETc, yield, and WP (Eq.5) in strawberry under different scenarios (soil types, irrigation and mulch materials).Relative yields (Y rel ; Eq. 6) and relative WPs (WP rel ; Eq. 7) 65 were also used to identify the best irrigation requirements to obtain high yields and WP under the scenarios.
where Y (t ha −1 ) is the simulated yield, Y m (t ha −1 ) is the simulated maximum yield, WP (kg ha −1 mm −1 ) is the simulated maximum WP among the scenarios, and ET (mm) is simulated evapotranspiration during the entire crop growth period.

Table 3 .
Validation a sudden rise in temperature from the second week of February (average temperature increased to 26.7 °C on 27th February from 18.2 °C on 10th February during the representative year 2001) causing thermal heat stress resulting in a higher rate of senescence compared to that predicted by the model.The model computed the aboveground biomass from Water Productivity (WP) and transpiration from an adjusted crop coefficient.
statistics for the canopy cover, aboveground biomass and soil water storage (over the 400 mm profile) in strawberry under different irrigation regimes and mulching during 2016-17 and 67 ; NM, SM, BM and JM are no-mulch, straw mulch, biodegradable plastic and jute agrotextile mulch, respectively].n, r 2 , RMSE, and d indicate number of observations, coefficient of determination, root mean square error, and the index of agreement respectively.

Table 4 .
Validation statistics for the fruit yields of strawberry under different irrigation regimes and mulching during 2016-17 and 2017-18.n, r 2 , RMSE, and d indicate number of observations, coefficient of determination, root mean square error, and the index of agreement respectively.